1. Field of the Invention
This invention relates to flow rate control of hydraulic pumps, especially those used in automotive power steering systems. The invention pertains particularly to an orifice fitting having an aperture connecting the pump discharge and a load such as a steering gear.
2. Description of the Prior Art
The flow control system of an automotive power steering system ideally increases the flow rate delivered to the steering gear linearly with pump speed over a low speed range extending from zero to about 700-800 rpm. Thereafter as pump speed increases, flow rate is held constant or nearly constant by diverting flow from the steering gear to a bypass port leading to the pump inlet.
Conventionally a flow control valve includes a spool slidable in a cylinder, a port connected to the pump outlet, a bypass port, a spring urging the spool to close the bypass post, an orifice connecting the pump outlet and the steering gear, and a passage connecting steering system pressure downstream from the orifice to an end of the spool. A pressure force develops on the spool due to this feedback pressure tending to combine with the spring force to close the bypass port. These spool forces are opposed by a force on the spool resulting from pressure upstream from the orifice tending to open the bypass port.
Therefore, as pump flow rate increases, the pressure differential across the orifice increases and the spool moves in the valve cylinder against the spring force to open progressively the bypass port. As the bypass port opens, system pressure decreases because flow is diverted from the steering gear directly to the pump inlet.
Flow to the steering gear can be reduced to a constant flow rate at the highest range of pump speed, in comparison to a higher constant flow rate at a lower speed range, by use of an orifice whose effective flow area is adjusted according to flow rate by a drooper pin. Conventionally a drooper pin is carried on the valve spool and includes at least two concentric areas of unequal size connected by a transition zone. The pin is drawn through the orifice aperture as the spool moves in the valve in response to differential pressure. The smaller pin area produces a smaller pressure drop for a given flow rate, the larger pin area produces a larger differential pressure across the orifice.
Feedback pressure on the valve spool is lower when the pressure drop is greater. Therefore, when the larger pin area enters the aperture, the bypass port is opened more fully than when the smaller area is in the aperture. Consequently, flow is diverted more fully from the steering gear to the pump inlet. A drooper pin permits multiple flow rates to the steering gear over ranges of pump speed.
However, a drooper pin requires precise dimensional tolerance control among the orifice aperture and drooper pin areas, and close correlation between the effective size of the orifice aperture, spool position and flow rate to the steering gear. Close tolerance machining is required. High cost and complexity in machining or otherwise forming the pin result necessarily from use of a drooper pin.
Various techniques for controlling operation of the flow control valve have been developed. For example, U.S. Pat. No. 4,289,454 describes a vane pump having two outlet ports, one port being closed after the flow rate exceeds a predetermined magnitude due to an increase in speed of the rotor. The excess fluid normally passing through one of the outlet ports is returned to the pump inlet to increase the fluid flow rate to the steering gear during high speed conditions.
U.S. Pat. No. 4,470,762 describes a pump having a control that bypasses flow from the pump between a cam ring and thrust plate. A spring opens the bypass passage and a pressure plate closes the bypass passage when system pressure rises. The pump control described in U.S. Pat. No. 4,470,764 includes a spring operating on a valve spool to open bypass flow and biased by system pressure to reduce bypass flow. In the vane pump of U.S. Pat. No. 4,470,765, output flow is partially bypassed through a flow control valve. The valve is operated by system pressure to close bypass passages as system pressure rises, thereby increasing flow to the power steering system.
More recently, power steering systems include electronically variable orifices that are opened and closed in response to vehicle speed and steering wheel speed so that the flow rate to the steering gear from the pump outlet is high when the required steering assist is high, particularly at low vehicle speed, and is low when the required steering assist is low, particularly at high vehicle speed and low steering wheel speed. An example of a power steering system controlled in this way is described in U.S. Pat. No. 4,473,128 in which a bypass valve directs a portion of the fluid flow from the pump to the steering gear in response to vehicle speed and angular velocity of the steering wheel. The position of the bypass valve is controlled by a solenoid, energized and deenergized on the basis of control algorithms executed by a microprocessor. The flow control valve described in U.S. Pat. No. 4,691,619 is also operated by a solenoid, which is energized and deenergized in response to vehicle speed. A pressure modulated slide valve is hydraulically piloted by a solenoid-operated valve. Fluid flow to the steering gear is controlled entirely hydraulically in response to vehicle speed and demand requirements represented by the steering gear input.
U.S. Pat. No. 4,485,883 describes a power steering system having a bypass valve controlling the flow rate of fluid directed from the pump outlet to the pump inlet and a constant flow valve for regulating the flow of bypass fluid. This control system reduced the flow rate to the steering gear during steering maneuvers at high speed and increases the flow rate at low speed and during parking maneuvers.
A similar object is realized with the power steering systems described in U.S. Pat. Nos. 4,561,561; 4,570,735. A vehicle speed sensitive valve operates to deactivate a conventional flow control bypass valve by eliminating differential force on the flow control valve at speeds greater than a predetermined value. U.S. Pat. No. 4,714,413 describes a power steering system of this type. Another control system of this type employing a solenoid-operated vehicle speed sensitive valve in combination with a conventional flow control bypass valve is described in U.S. Pat. No. 4,609,331.